Thymidylate-synthetase and various DNA-polymerases are target enzymes in the chemotherapy of cancer. Various 5-substituted pyrimidine nucleosides have been shown to be efficient substrates for these enzymes and also effective cancer chemotherapeutic agents. Thus our goals are: (1) to enhance the understanding of the chemical features of some 5- alkyl- and 5-hydroxyalkylpyrimidines, and to relate them to the demonstrated anticancer and antiviral activities and also to their enzymatic susceptibilities, (2) to develop routes for the synthesis of model compounds required for the above studies, and for their evaluation in the chemotherapy of cancer, (3) to contribute to the understanding of action of some antiviral agents and to develop chemotherapeutic controls for oncogenic viruses.